I. Field of the Invention
This invention relates to heat-resistant resin compositions and, more particularly, to a highly heat-resistant thermosetting resin composition which, when cured, exhibits excellent electrical and mechanical properties even at high temperatures above 150.degree. C.
II. Description of the Prior Art
Heat-resistant resins are often used for electrical insulating materials which are to be exposed to high temperatures. Among such heat-resistant resins are known maleimide resins obtained by reaction of amines which maleic anhydride. When cured to polymerize, these maleimide resins not only exhibit excellent heat resistance, but also provide a dense cured texture because the curing reaction is an addition reaction and does not yield any volatile low molecular weight by-product. However, these maleimide resins have low solubility in common organic solvents and require strongly polar high-boiling solvents such as dimethylformamide, dimethylacetamide and N-methylpyrrolidone in order to bring them into solution. This makes them hard to handle.
In addition, epoxy resins derived from phenol novolak and cresol novolak are also known as the heat-resistant resins that are widely used in practical applications. However, if these epoxy resins are exposed to high temperatures above 150.degree. C. for a long period of time, their mechanical properties and electrical insulating properties will be deteriorated to a remarkable degree. Thus, they are not suitable for use as electrical insulating materials which are to be used under such conditions.
Furthermore, it is also in current practice to blend the aforesaid maleimide resins with, for example, epoxy resins for the purpose of improving their properties. However, since the maleimide resins are poor in solubility with epoxy resins and the like, the resulting blends generally tend to form a precipitate in the vicinity of room temperature and, therefore, have the disadvantage of being difficult to use in solventless applications.